Social gaming star Zynga’s IPO last week casts the spotlight on its virtual goods business, which accounts for 95 percent of 2010’s $597 million in recognized revenue. The company’s broad appeal made me wonder, How do virtual currencies work today and where, outside of gaming, might they be effective? Right now, practical alternative payments systems like Facebook’s and PayPal’s are still based on cash, though consumers might like bartering and loyalty programs rolled into the mix.

Based on that survey, two features look critical for virtual currency systems: They should offer loyalty programs like frequent-buyer benefits, and the systems should work across different goods suppliers. The latter is one of the main reasons Facebook rigidly enforces Credits exclusivity for apps: The same accrued Credits can rent streamed videos or buy power-ups and game paraphernalia. The PayPal survey showed that a relatively high 36 percent of social gamers said they would be more likely to purchase virtual goods if they could earn loyalty credits for frequent purchases. And 22 percent said they would if they could use the same payment product for all games. Those rates were similar or higher for hard-core gamers as well. The frequent-buyer program angle was even more popular for other paid media like video (45 percent) and music (52 percent). Amazon and Apple should pay attention.

Companies offering new virtual currencies often promote two other features: micropayments under $1, which aren’t cost-effective with credit card fees, and the ability to base currency value on something other than money, including “bartering” user time and attention as well as other goods. No one has ever proven the value of micropayments. Failures include Beenz, Flooz, Millicent and others, though Flattr is still trying. That’s mainly because there aren’t that many things Americans buy that are worth less than a dollar. Music stores try to scrape a margin from 99-cent singles with prepaid PayPal accounts or by bulking up orders, and prices are moving to $1.29.

TrialPay is experimenting on Facebook and elsewhere with a variety of bartering options. Users can earn gaming goods for making purchases or watching ads, and they can trade in credits for coupons redeemable for real-world products like soda and cosmetics. Facebook is a promoter, and TrialPay claims to have 100 million users and 2,000 advertisers.

Jun Group recently raised $2.5 million to boost its ad serving system, which trades currency credits for viewing video ads. It claims a high 70 percent completion rate for viewing. Jun Group has worked with big brands like HBO and Frito-Lay.

Quora, the Q&A site, rewards behavior like answering questions and getting positive votes with credits that users can use to promote their own questions, so they have a better chance of being answered. The system reinforces expertise and participation, so it isn’t likely to migrate outside Quora. But other participative activities like reviews, recommendations and polling could apply similar techniques. Companies with consumer review systems like Amazon, Yelp and Angie’s List could use currencies to reward quality reviews, as determined by voting.

Yes, there is some evidence that users undervalue goods they have earned for actions. Users uninstall a lot of those apps, and Apple has banned trading goods for downloading an app, because developers were using the practice to game the App Store’s popularity ranking. But overall, barter-based currencies appear to be gaining momentum. If Zynga wanted to launch its new site with a bang, it would deploy a new virtual currency scheme that worked across third-party games and maybe even other products, support a frequent-buyer program and let users exchange their time and attention for credits.